The measurement of sound speed in a fluid as a function of both temperature and pressure is useful for the determination of accurate equations of state. Sound speeds in liquid water have been reported up to about 3.5 GPa, at temperatures up to about 100° C. using complex, custom-made measuring instruments requiring high-precision machining and fabrication, which also tend to be non-portable and largely inseparable from the test environment. That is, samples are introduced into the instrument rather than the instrument being moveable from one test environment to another. Applications requiring an apparatus for measuring sound speeds of liquids that is capable of performing high precision measurements, portable between test environments, mechanically rugged, and able to withstand high temperatures, include the characterization of Enhanced Geothermal Systems (EGS) or Hot Dry Rock (HDR) working fluids, as well as those in the oil industry. Sound speed varies with many physical parameters of a liquid such as temperature, pressure, and dissolved solid or gas content. Thus, when used with complimentary characterization tools, sound speed provides valuable information regarding the dynamics of a fluid system
Greater than 97% of the land area of United States is at temperatures of less than 250° C., at depths up to 10 km, making this a good high temperature target for operation of down hole characterization instruments. While the high pressures of such environments can be accommodated by incorporating appropriately thick walls in the device packaging, at the temperatures characteristic of down hole applications, fluids in geothermal systems are chemically harsh brines which are corrosive.
Swept Frequency Acoustic Interferometry (SFAI), where acoustic resonances of a fluid-filled cavity are measured over a range of frequencies, provides information from which the fluid sound speed, attenuation, and density can be determined. Currently available SFAI resonant cavities are delicate, room temperature instruments constructed using high purity, optically polished glass, which are excited using commercially available broadband transducers limited to operation near room temperature.